This invention relates to an optical pickup apparatus and, more particularly, to an optical pickup apparatus in which light is condensed on a recording medium by an objective lens, light reflected from the recording medium passes through the objective lens so as to be condensed thereby to irradiate detectors, and the objective lens is positionally controlled based on photoelectric outputs produced by the detectors.
In an apparatus of this type, problems are encountered in terms of improving the accuracy and reliability of focusing control and tracking control of the objective lens. A solution to these problems is closely related to the structure of an actuator which supports the objective lens.
In the prior art, known methods of controlling focusing include an astigmatism method, a critical angle method and a knife-edge method. With these methods, however, focus detection is performed by the distribution of an amount of light on a single split detector. Though the apparatus is simple in structure, disadvantages are malfunctions caused by scratches or contaminants on a disk surface, a large amount of influence of a groove, etc.
If, say, the aforementioned astigmatism method is implemented in an optical arrangement in which an optical path is split into two optical paths and the outputs of these two optical paths are inverted, the foregoing disadvantages can be eliminated by taking the difference between the two outputs. However, this expedient requires very fine focus adjustment in each of the optical paths. As a result, the labor involved in such adjustment is twice that of the prior art.
Moreover, if the optical pickup is used for a magneto-optical disk, splitting of the light path is performed by polarization. As a consequence, the effects of birefringence of the disk cannot be ignored. More specifically, if the amount of light on each optical path varies uniformly, no problems arise. In actuality, however, the influence of birefringence of the disk is dependent upon the angle at which light is incident upon the disk and becomes more pronounced as the angle of incidence increases. Accordingly, though not much influence is received near the optic axis of each optical path, the variation in the amount of light is great at the peripheral portions and, as a result, accurate detection of focusing cannot be performed with the prior-art method.
Further, in an apparatus of this type, a tracking servo operation is performed by applying a displacement (a lens shift) in the radial direction of the disk to an actuator (i.e., the objective lens). In the lens-shifted state, however, the center of the light condensed on the disk surface and the center of the luminous flux returned by reflection deviate from the center (optic axis) of the optical head. As a result, the irradiation pattern of the returned luminous flux causes this deviation to show up on the detector and appears as a so-called offset quantity in the output of the detector. This has an adverse effect upon the tracking servo operation. Accordingly, it is necessary to detect and cancel the offset by some method. In the prior art, a position sensor is separately provided to detect the amount of shift in the objective lens. However, separately providing the position sensor requires additional space and wiring and leads to a more complicated structure.
In the conventional apparatus, the objective lens is fixed to the actuator, which is driven in the focusing direction and radial direction of the disk. The conventional actuator is of two types, namely a shaft sliding type and a leaf spring type. The shaft sliding-type actuator has the following drawbacks: (1) resonance and a phase shift are caused by shaft backlash; (2) the empty weight of the set-up has an influence in the focusing direction; and (3) friction at the shaft impedes smooth displacement. With the leaf spring-type actuator, the disadvantages are as follows: (4) there is the possibility of causing vibration in directions other than the focusing direction and radial direction; (5) though the disadvantage (4) does not occur in a hinged arrangement, the provision of separate hinges for the focusing direction and track direction results in a complicated structure; and (6) the empty weight of the set-up has an influence in the focusing direction and track direction.